The invention relates to the improvement of the flow properties of waxy crude oils.
Dependent on the field of production, crude oils may contain considerable quantities of wax, e.g., from 6 to 20% by weight or more. This wax gradually separates when the oil is cooled to below a certain temperature. The coherence of the separated wax crystals in spatial structures imparts a certain stiffness to the oil. At sufficiently low temperatures the oil may even completely solidify. As will be explained below, the presence of crystallized wax in crude oil has a detrimental effect on the flow properties and handleability of the oil.
When crude oil is produced from a well which passed through strata of lower temperatures than that of the oil-bearing formation, the oil coming into contact with the cold wall of the well may stiffen, which interferes with its transport to the surface. If production is temporarily interrupted, the oil may even solidify completely, which involves serious problems when production is resumed.
When oil is stored in tanks which are not provided with heating facilities or insulation, the oil in contact with the cold walls and bottom of the tank will cool down and, as a result, may stiffen. This leads to difficulties in pumping the oil from the tank; in fact, considerable quantities of stiffened oil may thus remain behind, which reduces the effective capacity of the tank.
This problem acquires even greater importance during transport of waxy crude oil in unheated tankers, where the walls of the compartments are partly formed by the ship's hull which is in direct contact with cold sea water. Large quantities of stiffened oil remaining behind when the tanker is discharged, reduce the carrying capacity of the ship; in addition, subsequent cargoes of crude oil may become contaminated.
The poorer flow properties of the oil at lower temperatures will also considerably interfere with the transport of the oil through a pipeline, either by pumping or by flow under the influence of prevailing pressure or level differences.
When waxy crude oils are pumped through a pipeline, high flow resistances can build up, thus increasing pumping requirements. This can lead to high cost of transport, particularly in long-distance pipelines. If the resistance is very high, the available discharge pressure of the pump or the maximum permissible pressure determined by the strength of the pipe may be insufficient, so that the crude oil cannot be pumped.
If pumping is interrupted when the waxy crude oil is in the pipeline, the oil, which is often warmer than the surroundings, will cool down. The wax separated during cooling can freely form a spatial structure, which may extend over the entire cross section of the pipe and requires a very high pump pressure to be broken. If this pressure exceeds the available or permissible discharge pressure, transport cannot be resumed.
When a waxy crude is pumped through a pipeline as well as when it remains still, the oil may solidify on the cold pipe wall to form a deposit which remains behind. This reduces the capacity of the pipeline and entails the risk of contamination of subsequent batches of crude oil which have to be pumped through the pipeline.
Certain operations of crude oil refining, such as separation of water or sediment, for example by means of settling, centrifuging, filtration or coalescence, require the oil to be thin-liquid. If, owing to the presence of crystallized wax, the flow of the oil is insufficient, there is a possibility that these operations cannot be carried out at all or only to a limited extent. Good flow properties of a crude oil are desired not only for transport and storage, but also for many other reasons. In this connection, for example, mention may be made of sampling, the transmittance of pressure signals through narrow lines and the proper functioning of automatic equipment installed in refineries and along pipelines for purposes such as temperature and density measurements.
Thus it can be seen that the flow properties of a crude oil play a great part both during production and upon storage, transport and refining of the oil. It is therefore very important to minimize the adverse effect of wax on the flow properties of the oil.
To predict the flow behavior of a crude oil under operational conditions, laboratory-scale measurements of its pour point, which is regarded as characteristic of the flow behavior of the oil, is often carried out. The point is considered to be a criterion of the lowest permissible temperature during storage or transport or during a possible interruption of transport. Compounds are known which, when present in a crude oil, lower the pour point of that oil. For example, various homo- and copolymers containing aliphatic hydrocarbon side chains of at least 14 carbon atoms are disclosed for this purpose in Netherlands Application No. 6,709,453. British Pat. No. 1,154,966 discloses similar polymers as flow improvers in residual fuels.
The pour point of crude oil is usually determined according to a standardized procedure, involving cooling of the oil at a rate of about 0.5.degree.C/min (= 30.degree.C/hr). In actual practice, such as in pipeline transport or during shut-down of the pipeline, much slower cooling rates are frequently experienced, e.g., cooling rates of 5.degree.C/hr or less. It has been found that under these conditions the above-mentioned known polymers do not have the desired beneficial influence on the temperature at which the crude oil stiffens, and consequently the crude oil can no longer be easily transported.
A type of compound has now been found, which, when incorporated in a waxy crude oil, not only lowers the stiffening temperature at high cooling rates, but also at the lower cooling rates frequently encountered in the field.